1. Field of the Invention
The present invention relates to a dielectric composition which can be fired at a temperature of below 900° C. while having a high dielectric constant and quality factor, and more particularly, to a composition with a dielectric constant in the range of 20–40 and a quality factor (Qxf) of above 10,000 GHz. This type of composition can be effectively applied to constitution of a part of a ceramic multi-layer packaging as a type of resonator such as a filter or an antenna.
2. Description of the Background Art
Development of an information and telecommunication system and related parts is largely focused on the trend of high frequency, compactness and high performance.
Especially, in order to make parts compact, a passive integration technique is required to make a passive device one module, for which many researches and development are ongoing.
Various techniques have been invented to implement a multi-layer packaging by using a ceramic material having a low dielectric loss value at a microwave band. In most cases, however, in order to fire the ceramic material, a firing process is necessarily performed at a high temperature of above 1,300° C. Thus, in order to form a conductor line inside the ceramic packaging with a multi-layer stacked structure, precious metals such as Pt or W have been used. Those precious metals, however, has problems that they are costly and have bad electrical properties because of a low electric conductivity.
Recently, a research is being actively conducted on a multi-layer ceramic packaging using the internal electrode such as Ag or Cu with an excellent electrical conductivity, instead of using the electrode such as Pt or W.
That is, a ceramic substrate with a low dielectric loss value and an AG/Cu electrode are stacked as a multi-layer and co-fired to obtain a high density three-dimensional line substrate with superior electrical properties. In such a case, in order to minimize a signal delay, preferably, the ceramic substrate has a low dielectric constant, and in order to minimize an electrical loss, the dielectric loss value is preferably small.
In addition, in order to co-fire it with the Ag electrode, the ceramic composition should have a firing temperature of below 900° C.
Relevant techniques mostly show a combination of B2O3—SiO2 system glass frit and Al2O3 filler. In this case, the ceramic substrate usually has a dielectric constant in the range of 4–10.
The conventional techniques are directed to ceramic multi-layer packaging utilized only with a simple three-dimensional wiring substrate. Lately, however, as various types of passive parts are implemented inside the multi-layer ceramic packaging, advanced from the simple wiring substrate, a necessity of adding diverse functions to the packing emerges.
In particular, in order to implement a filter or an antenna in the resonator type inside the multi-layer ceramic packaging, a composition with a high dielectric constant is required. In order to control a distributed circuit element such as the filter or the antenna in the resonator type to a suitable size, a length of an effective wavelength needs to be reduced.
Currently, a microwave band is in the range of 1–300 GHz, and in this frequency range, a dielectric constant range required for obtaining a length of the most suitable effective wavelength for implementing a device is 20–100.
In addition, it is preferred that the quality factor (Qxf) has a value higher than 1,000 and a temperature coefficient of resonant frequency is below ±20 ppm/° C.
Dielectric compositions with a dielectric constant of 20–100 and excellent microwave properties are ZrO2—SnO2—TiO2, MgTiO3—CaTiO3, BaO—La2O3—TiO2, BaO—TiO2 system.
These ceramics have a high quality factor at the microwave band (>5,000 GHz) but their firing temperature is mostly 1,300° C., high. Therefore, in order to make them a composition for packaging which can be fired together with the Ag/Cu electrode at below 900° C., a sintering additive should be added to lower the firing temperature.
U.S. Pat. No. 5,872,071 discloses lowering the sintering temperature to about 1,000° C. by adding a sintering additive in a form of BaCuO2—CuO in the range of 0.1–50 wt % to a ZrO2—SnO2—TiO2 composition with a dielectric constant of about 40. In this case, a dielectric constant is 35–40 and a quality factor (Qxf) is 7,000–35,000 GHz at 7 GHz.
U.S. Pat. No. 5,616,528 presents an example that up to 10 wt % of glass frit in the form of B2O3—Li2CO3 is added to a MgTiO3—CaTiO3 dielectric, which is then fired at a temperature of 950° C. In this case, a dielectric constant is in the range of 19–24.
U.S. Pat. No. 5,994,253 presents an example that 5–15 wt % of ZnO—SiO2—B2O3 system glass composition is added to a BaTiO3—TiO2 dielectric, which is then fired at a temperature of 900–940° C. In this case, a dielectric constant is between 60 and 90 and a value ‘Q’ is 1,000–1,500.
The above listed patent articles present the compositions which has the dielectric constant of 20–100 and can be fired at a low temperature, but their sintering temperature is about 900–1,000° C., which is high.
Though a melting temperature of Ag is 950° C., Ag is diffused into a neighboring dielectric at above 900° C., causing problems that the electrode is changed in its form and electric conductivity is degraded.
Therefore, in order to co-fire it with the Ag electrode, the firing temperature of the ceramics is preferred to be lowered down.